This invention relates generally to optical recording. More particularly, this invention relates to a method and apparatus for reading optically encoded data from a disk.
Personal computers typically connect to an optical disk drive such as a CD-ROM to read data from a compact disk. On the compact disk, data is stored in the form of pits and lands patterned in a radial track. The track is formed in one spiral line extending from the inner radius of the disk to the outer edge. A pit is a location on the disk where data has been recorded by creating a depression in the surface of the disk with respect to the lands. The lands are the areas between the pits in the tangential direction. The reflectivity of the pits is less than the reflectivity of the lands. To store audio or digital information, the length of the pits and lands are controlled according to a predefined encoding format.
When reading information from the disc, light from a laser beam is directed onto the track and the light beam is reflected back to a photo-sensor, such as a photo-diode. Since the pits and lands have different reflectivity, the amount of reflected light changes at the transitions between the pits and the lands. In other words, the encoded pattern of the pits and lands modulates the reflected light beam. The photo-sensor receives the reflected light beam, and outputs a modulated signal, typically referred to as an RF signal, that is proportional to the energy of the light in the reflected light beam.
In FIG. 1, the relationship of the RF signal to the pits and lands is shown. A smaller pit or land decreases both the period and the amplitude of the RF signal. The RF signal has opposite polarity in the pits and lands.
One encoding format used in optical disk systems is eight-to-fourteen modulation (EFM). EFM reduces errors by minimizing the number of zero-to-one and one-to-zero transitions. In other words, small pits are avoided. In EFM, the data signal includes no less than two zeros and no more than ten zeros between logical transitions at the pit edges. A zero is indicated by no change in the energy reflected beam for at least two clock periods. A one is indicated by a change in the energy of the reflected light beam, that is, a pit edge. Applying the EFM encoding rules, a pit or land will have a length corresponding to the amount of time for at least two, and up to ten, clock periods and the electronics will output a corresponding voltage as shown in FIG. 1.
In an optical disk drive, an optical head assembly includes the photo-sensor, a tracking actuator and a lens. The photo-sensor is mounted on a sled. The lens is mounted on the tracking actuator which is mounted on the sled. The lens is positioned between the photo-sensor and the disk to transmit the light beam from the laser onto the disk surface and to transmit the reflected light beam to the photo-sensor. The sled and tracking actuator position the photo-sensor and lens with respect to the spiral track. The sled is driven by a sled motor that positions the optical head assembly radially across the disk. The tracking actuator is a voice coil motor that positions the lens within the limits of the sled. Because the geometry of the photo-sensor is large with respect to a single track, the lens can be positioned within a range of tracks and the photo-sensor can properly detect the RF signal.
Information is written on predetermined regions of the track and the lens is positioned over a target region of the track. The spiral track is written within a main storage area of the disk. The DVD disks store any one or a combination of video, audio and data signals in the main storage area.
A search is performed to position the optical head assembly over a target region of the spiral track. During searching, track crossings will be detected as the lens moves radially across the spiral track thereby providing position information in terms of a number of xe2x80x9ctracksxe2x80x9d crossed with respect to a given position on the disk. During rough searches, the prime mover of the optical head assembly is the sled motor. On fine searches, the prime mover of the lens is the tracking actuator.
DVD disks have another area, within the disk hub, in which a bar code is written to store disk identification information. This area is referred to as the burst cutting area (BCA). The burst cutting area is part of the DVD standard. The bar code stored in the burst cutting area is referred to as a BCA code. In the burst cutting area, the BCA code is recorded in a lead-in area near the inner ring of the disk. The BCA code is not recorded in the main storage area. The BCA code stores from twelve to 188 bytes in sixteen-byte increments and allows serial numbers to be associated with disks. The BCA code also aids in piracy protection because it can include an encryption/decryption key, thereby making BCA code suitable for use with the Internet. Using the encryption/decryption key, a disk having multiple programs can be customized for partial sale. In a partial sale of a disk storing a set programs, the purchaser pays for a subset of the programs on the disk. The purchaser is provided with appropriate decryption keys that allows the purchaser to play the subset of programs for which they paid. To read the BCA code, the prime mover of the sled is pushed against an inner stop and the secondary lens positioning tracking actuator runs freely within the carriage while reading the BCA code.
To keep the lens properly positioned over the spiral track in the main storage area, the electronics generates a track error signal from the light received by the photo-sensor that is reflected from the spiral track. The photo-sensor has a size and position that is a function of the ideal center position of the lens with respect to the spiral track.
In the burst cutting area, there is no track error signal to control the positioning of the tracking actuator with respect to the sled. One method samples a track error signal when positioned over the spiral track, and uses the sampled track error signal to position the lens in the burst cutting area while reading the BCA code. A sample-and-hold circuit maintains the sampled track error signal, thereby treating the burst cutting area as a large defect area.
On a DVD disk, the track pitch of the spiral track in the main storage area is 0.00074 millimeter (mm) while the width of the burst cutting area is 1.15 mm. This difference in track width increases the difficulty of controlling the lens position in the burst cutting area because no meaningful track error signal can be generated. In addition, because the burst cutting area is very wide as compared to the track width of the spiral track, some drives position the lens by moving the sled via a rough search operation. Since no track error signal is present in the burst cutting area, it is difficult to precisely determine the sled position with respect to the burst cutting area.
Reading the BCA code without precise control of the lens position is not reliable. Disk drives may have marginal tolerances with respect to the innermost lens position and repositioning the lens may be desirable. For example if the lens is positioned at the edge and not completely over the burst cutting area, errors may result when reading the BCA code. In addition, if the lens is positioned on the edge of the burst cutting area, disc run-out may cause errors thereby degrading the readability of the BCA code. The burst cutting area may also have defects that produce errors when reading the BCA code.
In view of the foregoing, it would be highly desirable to provide a method and apparatus to improve the readability of the BCA code.
A method and apparatus provide positive lens position control when reading information from a disk that stores data using a first and a second encoding format. When reading the data encoded in the second encoding format, the lens is maintained in a substantially stationary radial position with respect to the disk. When the data encoded in the second encoding format has an error, the lens position is changed to find a better position for reading the data.
In particular, positive lens position control is provided when reading BCA code on DVD discs. When reading the BCA code, the lens is maintained in a substantially stationary radial position with respect to the disk. When the BCA code has an error, the lens position is changed to find a better position from which to read the BCA code.
A method includes reading information recorded on a disk having first and second encoded data areas. The second encoded data area uses a different encoding format from the first encoded data area. A second encoded data area position signal is generated based on a track centerline signal and a second encoded data area position reference signal. The second encoded data area position signal drives a tracking actuator.
In another aspect of the invention, a circuit reads information recorded on a disk having first and second encoded data areas. The second encoded data area uses a different encoding format from the first encoded data area. A track centerline circuit generates a track centerline signal. An adjustment circuit generates a second encoded data area position signal from the track centerline signal and a second encoded data area position reference signal.
In yet another aspect of the invention, computer readable program code precisely controls the lens to read the second encoded data in the second encoded data area.
In this way, the readability of the data encoded in the second encoding format, such as BCA code, is improved by positively controlling the motion of the tracking actuator and the sled.